1. Field of the Invention
This invention relates to radiation detectors and, more specifically, to such detectors which are based on semiconductor principles.
2. Prior Art
Semiconductor-based radiation detectors generally have a single-crystal substrate with a p-n junction or a Schottky junction with an inverse bias applied to the depletion layer which occurs at the junction. When radiation strikes the depletion layer such radiation is detected by detecting the increase in electron-hole pairs which occurs in the region where the radiation is incident. This technique is effective in detecting radiation of the .alpha., .beta., .gamma., and/or x-ray types.
When it comes to detecting neutrons, which by definition have no charge, there is no electrostatic influence on the Coulomb fields of the orbital electrons and atomic nuclei so the electron-hole pairs relied upon in the conventional semiconductor-based radiation detector do not appear as a result of incident neutron bombardment. Thus, the conventional semiconductor-based radiation detector cannot be used to detect neutrons. With this problem in mind, neutron detection has been approached from the standpoint of detecting charged particles produced by the reaction between the incident neutrons and the atomic nuclei. Such reaction produces free protons as a result of elastic scattering and nuclear fission. A prior art neutron detector based on this principle is shown in FIG. 1. In FIG. 1, a single-crystal silicon substrate detector-element 10 is housed in a sealing vessel 12 comprising a vessel body 14 and a cover 16, the purpose of which is to produce an hermetic seal of the vessel. Lead wire 18 is led out from one surface of semiconductor-based detector element 10 and through hermetic packing element 20. In the embodiment of FIG. 1, showing the prior art, the remaining lead wire 22 is grounded to the vessel body 14 which is metallic and, therefore, electrically conductive in character. The lead wire 18 is coupled to an indicating device, not shown, for indicating the presence and level of neutron incidence. A pipe 24 is hermetically sealed in cover 16. In operation, .sup.3 He is introduced into vessel 12 by way of pipe 24 from a source of such gas, not shown. This type of semiconductor-based neutron detector relies upon the reaction and the elastic scattering which result with respect to .sup.3 He and the incident neutrons, according to the following equation EQU .sup.3 He+n.fwdarw..sup.3 He+P+765 KeV (1)
where P is protons. It can be seen from that equation that the incidence of neutrons on the .sup.3 He produces .sup.3 He+protons and a nuclear energy of 765 KeV. More specifically equation (1) shows that the incidence of neutrons on a .sup.3 He produces .sup.3 He and the following components: (1) reaction-generated nuclear energy to which an electron kinetic energy of 765 KeV is ascribed when the neutrons are reactive, (2) the reaction generated nuclear energy caused by the thermoneutron background, and, (3) protons produced by the elastic scattering caused by the incidence of neutrons upon .sup.3 He.
In the detector of FIG. 1, however, charged particles generated in the .sup.3 He gas must pass through the gas in order to reach the detecting element and, hence, the detection efficiency is decreased. The resolving power of the detector and and its associated indicating equipment is consequently limited. Further, the vessel 12 must be very strong because the .sup.3 He gas is introduced into the vessel at a pressure of 1 to 5 atmospheres. Further, the detector of FIG. 1 requires various ancillary equipment, such as pipes for introducing the .sup.3 He gas into vessel 12. It is also necessary to have a gas flow rate regulator. Thus the equipment of FIG. 1 is not portable nor is it easy to utilize other than in a laboratory.
Therefore, it is a primary object of the present invention to provide a compact, lightweight semiconductor-based radiation detector which is highly portable and still also highly effective in detecting neutrons.